Sporulation of the yeast Saccharomyces cerevisiae offers the opportunity to study the control of gametogenesis both genetically and biochemically. The objective of the proposed research is to define the mechanisms which govern the transition of a cell from mitotic growth to meiosis and cell differentiation. The role of the sexual mating type locus, which must be heterozygous for sporulation to occur, will be examined in detail, using mutants of the mating type alleles which are suppressible by a dominant, temperature-sensitive amber suppressor. Diploids containing these alleles and the suppressor are thus temperature-sensitive for sporulation. In addition mutants affecting interconversion of mating type in homothallic strains will continue to be studied. The mechanisms by which the synthesis of nucleic acids -messenger RNA, ribosomal RNA and DNA - differs from their control during vegetative growth will also be examined. The half-life of messenger RNA during sporulation will be compared with that during growth, and differences in the extent of polyadenylation of mRNA will be followed. The genetic and physiological control of rRNA, including the accumulation of a species unique to cells able to sporulate, will be pursued. The genetic and biochemical events controlling the appearance of sporulation specific 20S RNA and an associated protein will also be investigated. Finally, new experimental approaches to examine in detail the synthesis of DNA during meiosis and the period of extensive meiotic genetic recombination will be developed. These include the improvement of synchrony of premeiotic DNA synthesis and isolation of strains able to incorporate the density label bromodeoxyuridine into DNA.